This invention relates to the detection of ice on the surface of an aircraft. More specifically it relates to a method and apparatus for the measurement of the thickness of ice on an aircraft surface.
It is well known that ice buildup on aircraft is a potentially serious safety issue. Ice buildup, in addition to increasing the weight of the aircraft, can interfere with the aerodynamic characteristics of the aircraft. In particular, ice buildup on wings changes the effective airfoil shape of the wing, and can possibly cause the flow over the wing to become turbulent. This reduces lift, increases drag, and makes the plane more difficult to control.
There are three types of ice that can form on an aircraft: rime, clear, and mixed. Rime tends to form from water that freezes as it hits the plane. Thus, rime generally forms on the leading edge of wings and tails. It is thicker than clear ice, as it traps air in it as it freezes. Clear ice does not entirely freeze as it hits. A portion of the water freezes on the initial contact surface, and then the rest freezes as it flows back over the aircraft. Clear ice has a denser buildup on the leading edge, and more buildup on the rest of the aircraft. Mixed ice is a combination of clear and rime ice. Clear ice can be the most dangerous, as most ice detection and deicing systems are installed at or near the leading edge of the wings and tail. Therefore, clear ice can buildup without the pilots realizing it, and it cannot be removed until the plane lands.
Various devices have been developed to detect the presence of ice on a surface such as an aircraft member. Often, they include locating a part of the detector on the outside surface of the aircraft. One example is shown in U.S. Pat. No. 5,585,551 (Johansson et al.). In the invention disclosed in this patent, a wire is routed through holes in the aircraft skin, along the outer surface, and back through holes to the inside of the aircraft. The problems with this system can include: weakening of the structure by drilling holes in the surface, adverse aerodynamic consequences by placing holes and wire on the surface, and the inability of the system to detect the thickness of the ice.
Other systems have attempted to avoid aerodynamic problems by mounting the apparatus within the aircraft. U.S. Pat. Nos. 4,604,612 (Watkins et al.) and 4,461,178 (Chamuel) disclose inventions that connect transducers to the inside surface and measure the effect on a wave that propagates down the surface from a transmitter to a receiver. These devices involve mounting the transmitter and receiver a significant space from each other such that the wave will propagate from one to the other. This reduces the locations on an aircraft where the transducers can be mounted. The effect of the ice on the wave will vary according to the thickness of the ice, which can vary along the path from the transmitter to the receiver. Thus, these methods are more effective at detecting the presence of ice, rather than measuring the thickness of ice buildup.
Methods to measure the thickness of ice have been disclosed, such as those discussed in U.S. Pat. Nos. 5,507,183 (Larue et al.) and 4,628,736 (Kirby et al.). Larue et al.""s invention includes a transmitter and receiver on a block mounted flush with the aircraft surface. It reflects an ultrasonic wave off the face of the block facing the outside. If there is ice on the surface, an additional reflection from the ice/air interface is received after the reflection from the block/ice interface. Measuring the time delay and knowing the speed of sound through ice enables calculation of the thickness. Kirby et al.""s invention uses one transmitter/receiver mounted flush with the surface to send ultrasonic waves out from the surface. If ice is present, it measures the time delay between the emission and reflection to find the thickness of the ice. Both of these devices are mounted flush to avoid interfering with the aerodynamics of the aircraft. However, this type of device can be more difficult to manufacture than a device mounted inside the surface. In addition, holes in the aircraft member""s surface to accommodate these devices may reduce the structure""s ability to bear loads.
Clearly, then, there is a longfelt need for an ice detection apparatus that can measure the thickness of ice on the aircraft surface which requires only minor structural changes to the aircraft to accommodate installation.
The present invention broadly comprises a method and apparatus for measuring the thickness of ice on an aircraft member. In the preferred embodiment, the apparatus is mounted beneath an aircraft member surface. The apparatus includes transducers for transmitting ultrasonic signals through the aircraft, member and ice at predetermined applied voltages and frequencies. The apparatus measures the current and current phase angle, relative to the applied voltage, associated with each transducer and frequency. The apparatus calculates the resistive component of the impedances of the transducers which will peak at frequencies dependent upon the thickness of ice accumulation on the aircraft member. The impedance will peak where the combined thickness of the aircraft member and the ice is an odd multiple of a quarter wavelength of the transmitted signal. The apparatus then computes the ice thickness by determining those frequencies where the impedance is a peak, calculating the thickness of the ice and member based upon the determined frequencies and their wavelength relationships, and subtracting the member thickness. The invention also broadly comprises an apparatus and a method for the detection of ice on an aircraft member.
A general object of the present invention is to provide an apparatus and method for measuring the thickness of ice on an aircraft surface.
Another object of the present invention is to provide an apparatus and method for detecting the presence of ice on an aircraft surface.
It is a further object to provide an apparatus that can be mounted inside the surface of the aircraft.
It is still a further object to provide an apparatus that is small enough in size such that it can be mounted inside small surfaces of the aircraft, so that the thickness of ice can be measured in the maximum number of locations.
These and other objects, features and advantages of the present invention will become readily apparent to those having ordinary skill in the art upon a reading of the following detailed description of the invention in view of the drawings and claims.